I use beertools pro on the mac. I'm fairly satisfied with it; it's overly complex and has a few niggly issues, but overall it does a good job at predicting things like strike temp and so on, once you have calibrated everything.

You can also use BeerPal for the iDevice. It costs like $0.99. I bought it and use it to mark down recipes when the inspiration strikes me. It also does a decent job at strike water temp and sparge volume predictions.

Finally, there's also the rackers calculators linked earlier in this thread. They're pretty good, but the formula there just assumes a 3F drop from the thermal mass of your tun. Not that big of a deal, especially once you've brewed a few times, as you'll quickly figure out your own fudge factor.